set MACOSX_DEPLOYMENT_TARGET to whatever detected during translation

disutils yields an error if you set it to something other than that
configured, and I tested that error

--HG--
branch : macosx-deployment-target-2.7
1 job for branch/macosx-deployment-target-2.7 in 7 minutes and 18 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #109251
check_rpython_annotations

00:07:18